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and on the sides. It is divided into two parts, the larger above, the smaller below; each section is provided with

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double doors. The lower chamber is only used when moist air is required in the thermostat, a saucer of water being

placed inside. The doors are made of glass, the outer one being covered with a piece of felt which can be removed.

The space between the walls of the box is filled with water; distilled water being used to prevent furring. The thermostat is provided at one side with a water gauge carrying a two-way cock, which allows the water either to enter the gauge or to be drawn off. In the top is an opening in connection with the space between the double walls of the box; this opening is used for filling the space with water, and, after this has been done, for holding a thermometer which passes through a cork and has its bulb dipping into the water. On the opposite side (to the right in the sketch) there is a similar opening to which a thermoregulator is fitted (see p. 46). There is a third opening, which is connected with the inside of the thermostat; a thermometer is placed in this by which the temperature of the chamber containing the cultures may be read. There are two other openings also leading to this chamber; they act as valves and are provided with caps which can be closed either completely or partially. Lastly there is an opening to which a hygrometer may be fitted. In the thermostat there are several movable shelves, which are perforated to allow of free circulation of air. Thermometers are required here also.

The heating is done by means of gas Hames which are placed under the apparatus, as may be seen from the figure. The water is thus warmed, but in order to keep the temperature constant the thermoregulator is indispensable.

Quite a large quantity of water is used in such a thermostat, about 43 litres being required for a thermostat 40 cm. high, 50 cm. broad, and 25 cm. deep.

Two gas fames are used so as to reach the desired temperature quickly, but this process may be made still shorter if the thermostat is filled with water previously

heated. The regulator is adjusted by being warmed in a water bath of the desired temperature before it is fitted to the thermostat.

Besides the above form there are numerous others. Muencke (Berlin) and Altmann (Berlin) also make good thermostats.

Panum's Thermostat.-In many investigations it is important to have at the same time a large number of chambers at different temperatures. Panum's thermostat (see Fig. 15), which has a series of constant temperatures, realises the required condition. We will describe in what follows its construction along with the improvements which this apparatus has undergone in the Carlsberg laboratory.

This thermostat consists of three cupboards soldered together which are designated in the accompanying Fig. 15 with the letters A, B-C, and D. The reason that it is not made of one cupboard divided into three parts is that A and D require more repairing than the middle one B-C. The former have therefore to be made separate from the latter. The inner dimensions of each of the compartments A, B-C, and D is about 63 cm. in length, breadth and depth. The first compartment, A, is double-walled, and is made of tinned copper; the jacket, c, is filled with distilled water, which is kept at the required temperature by a gas lamp, a; the gas flame is controlled by means of a regulator, b, which dips into the water through a tubular opening. This gas lamp is placed under a projecting wing, d', of the external copper case; this wing is also tinned and in addition covered with asbestos paper, excepting the part immediately above the lamp which is thickened by the addition of another copper plate. As the wing is burnt through in course of time it is not put into permanent connection with the outer copper case but joined to it by screws and flanges with rubber packing so that it can be replaced when necessary.

. There is also another tubular opening (not shown in the figure) at the top of the compartment A, and in front of

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Fig. 15.--Section through Panum's Thermostat. (The significance of the letters is explained in the text.)

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that holding the regulator, and through this a thermometer dips into the water to register its temperature. Water is

run in through one of the above openings, and can be run off at the stopcock, d”, situated on the wing.

The space in the compartment, A, is divided into two parts, 1 and 2, by a partition ; ledges, on which loose shelves can be laid, are fixed to the walls at various heights. There is an opening in the roof for a thermometer, e, to give the temperature of the air in the space A.

The other two compartments, B-C and D, are made of tinned sheet iron, the compartments D, and C8 being carefully painted all over with red lead to resist damp due to the cooling in D. B-C is divided into two compartments B and C; each of these being again divided into three spaces (3, 4, 5, and 6, 7, 8), the walls of which are also provided with ledges for fitting up several stages. These are all made of tinned sheet iron. In compartment C8 there is a bent metal strip, f, soldered along the bottom against the wall next to D so that the condensation water which forms on this wall can pass over the strip and run into a long narrow box placed below.

The last compartment, D, is an ice box consisting of an outer and inner receptacle, the latter of these being cooled by water trickling down over it from a mass of ice resting on a strong grating, g. To distribute the water the inner receptacle has its roof sloping to the sides and to the back. The water is run off through the opening, h; in this is a cork passing through which there is a tube, i, forming a water trap; the water is caught in a vessel, k, placed beneath. The cork must be taken out every day in order to remove dirt coming from the ice. There is a movable trap, 1, fitted to the top of D9, in which condensation water collects and is removed.

The whole apparatus is completely surrounded by a layer of felt, m, 8 cm. thick, and enclosed in a tight wooden box, o.

The ice holder is closed by an iron lid, above which is

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